A control device that controls an industrial system such as a steel plant, a paper manufacturing plant, or a petrochemical plant and a public system such as a water and sewage system receives input data related to control of an input and output device from the input and output device which is a control target external device with the aid of an I/O interface and temporarily writes the input data to an input register which is a memory area for the input data via a system bus. The control device writes the result of an arithmetic process performed on the input data stored in the input register to an output register which is a memory area for the output data as output data. After that, the control device transmits the output data stored in the output register to the input and output device with the aid of the I/O interface.
By the way, the control device performs batch input and output that the control device writes all items of input data newly received from the input and output device to the input register before an arithmetic process is performed on the input data stored in the input register and transmits all items of output data stored in the output register to the input and output device after the arithmetic process is performed on the input data stored in the input register. That is, the control device writes the input data received from the input and output device to the input register even when there is no change in the input data received from the input and output device and transmits the output data stored in the output register to the input and output device even when there is no change in the output data stored in the output register.
However, the number of items of data transmitted and received at a time between the control device and the input and output device is approximately 1,000 items in a medium-scale industrial or public system and is approximately 100,000 items in a large-scale industrial or public system. Therefore, batch input and output takes lots of time and it is difficult to shorten a control cycle of controlling the industrial or public system.